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Date:   Wed, 5 May 2021 22:22:18 +0000
From:   Joseph Myers <joseph@codesourcery.com>
To:     David Laight <David.Laight@ACULAB.COM>
CC:     'Florian Weimer' <fweimer@redhat.com>,
        "Alejandro Colomar (man-pages)" <alx.manpages@gmail.com>,
        linux-man <linux-man@vger.kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Greg KH <gregkh@linuxfoundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        GCC <gcc-patches@gcc.gnu.org>,
        "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>,
        glibc <libc-alpha@sourceware.org>, bpf <bpf@vger.kernel.org>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>
Subject: RE: [RFC v2] bpf.2: Use standard types and attributes
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On Wed, 5 May 2021, David Laight via Libc-alpha wrote:

> > __u64 can't be formatted with %llu on all architectures.  That's not
> > true for uint64_t, where you have to use %lu on some architectures to
> > avoid compiler warnings (and technically undefined behavior).  There are
> > preprocessor macros to get the expected format specifiers, but they are
> > clunky.  I don't know if the problem applies to uint32_t.  It does
> > happen with size_t and ptrdiff_t on 32-bit targets (both vary between
> > int and long).
> 
> uint32_t can be 'randomly' either int or long on typical 32bit architectures.
> The correct way to print it is with eg "xxx %5.4" PRI_u32 " yyy".

C2X adds printf length modifiers such as "w32", so you can use a 
friendlier %w32u, for example.  (Not yet implemented in glibc or in GCC's 
format checking.)

-- 
Joseph S. Myers
joseph@codesourcery.com